kernel: introduce supervisor-only stacks

These stacks are appropriate for threads that run purely in
supervisor mode, and also as stacks for interrupt and exception
handling.

Two new arch defines are introduced:

- ARCH_KERNEL_STACK_GUARD_SIZE
- ARCH_KERNEL_STACK_OBJ_ALIGN

New public declaration macros:

- K_KERNEL_STACK_RESERVED
- K_KERNEL_STACK_EXTERN
- K_KERNEL_STACK_DEFINE
- K_KERNEL_STACK_ARRAY_DEFINE
- K_KERNEL_STACK_MEMBER
- K_KERNEL_STACK_SIZEOF

If user mode is not enabled, K_KERNEL_STACK_* and K_THREAD_STACK_*
are equivalent.

Separately generated privilege elevation stacks are now declared
like kernel stacks, removing the need for K_PRIVILEGE_STACK_ALIGN.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/linker/linker-defs.h

@@ -245,6 +245,8 @@ extern char _ramfunc_rom_start[];
 #ifdef CONFIG_USERSPACE
 extern char z_priv_stacks_ram_start[];
 extern char z_priv_stacks_ram_end[];
+extern char z_user_stacks_start[];
+extern char z_user_stacks_end[];
 #endif /* CONFIG_USERSPACE */
 
 #endif /* ! _ASMLANGUAGE */